In the past, downhole tools have been positioned in a desired location by a variety of different ways. A tubing string can be assembled at the surface with the tool at the bottom for accurate placement of the tool at the desired location. Alternatively, coiled tubing can be used as well as mechanical cables or an electric line which combines the feature of mechanical support for the tool as well as the possibility of conduction of power control signals and data to and from the surface to the electrical components in the tool downhole. One of the disadvantages of using electric line is that special equipment needs to be provided for use of the electric line. In many cases, particularly offshore, space is at a premium and it is difficult to find a suitable location for the surface equipment needed to run the electric line. Additionally, the use of an electric line, or wireline, requires not only rental of the wireline equipment but also a wireline crew to operate the equipment. It is far more desirable from an operator's standpoint to use standard rig equipment to position a downhole tool for proper downhole actuation.
Some types of downhole tools have valves which require movement to selectively equalize pressures in two isolated zones which could vary substantially prior to valve actuation. Typically in these layouts, substantial amounts of energy are required to trigger valve movement. In these prior designs, the valve member would be actuable by a solenoid or a drive power screw or some other means that was connected to a downhole power source. However, due to the high power requirements and the limited space available for the downhole tools, problems arose in being able to locate within the tool a power supply of sufficient energy to actuate the valve element in view of the great disparity in pressures from two different zones across a valve member.
As previously stated, the power could come from the surface to the final control element, such as a solenoid valve located in the tool. However, such an arrangement would require the use of a wireline and the necessary incremental cost for having a wireline crew and equipment at the rig site. When using a purely mechanical support mechanism which permits rapid placement of the tool at the desired depth, such as a cable support operated from the rig drawworks, there no longer is the ability to provide the power supply from the surface to the tool located downhole. The apparatus of the present invention illustrates a tool that can provide selective communication between two pressure zones of differing pressures, based on a low-power power supply located within the tool which is actuable from the surface by a variety of nonelectrical means. Signals may be sent from the surface mechanically through motions or impacts imparted to either a support cable or a tubing string. One such method of transmission of such signals is disclosed in U.S. Pat. Nos. 5,226,494 and 5,343,963, as well as pending applications commonly assigned Ser. Nos. 08/071,422, filed Jun. 3, 1993 (Owens), and 07/751,861, filed Aug. 28, 1991 (Rubbo), which involves production of acoustical signals which pass through a conduit wall and are detectible by a strain gauge assembly on a downhole tool, to generate a signal for actuating the downhole tool using a downhole energy source. Signals can also be acoustically transmitted through the wellbore fluids where they are sensed at the tool downhole. Regardless of how the signal is transmitted, it is received at the tool where the control circuitry closes a circuit. In the preferred embodiment, the valve member is temporarily retained by a fastening mechanism, which in the preferred embodiment is a Kevlar.RTM. cable, wrapped with a heating element. When the control circuit is actuated to provide power to a heating element, the Kevlar.RTM. cable breaks and the differential pressure across the valve member actuates the valve member, thus opening fluid communication between what had previously been two zones isolated from each other at different pressures. Thereafter, once the valve member has shifted and various seals have moved, allowing a flow opening to be created between a zone of high pressure and a zone of lower pressure, flow through the tool is initiated. This flow can be used to move a piston or a setting sleeve, making the apparatus particularly useful in setting packers, as will be described below.